1. Field of the Invention
The present invention relates to removal of MAPD from hydrocarbon streams. More particularly the invention relates to a process wherein the MAPD is converted to valuable propylene by selective hydrogenation. More particularly the invention relates to a process where one of the hydrogenation reactors is a distillation column reactor wherein the propylene is concurrently separated from a stream containing unconverted MAPD.
2. Related Information
Methyl acetylene/propadiene (MAPD) is not a compound but covers the unstable compounds methyl acetylene and propadiene which may be depicted as follows: 
The MAPD compounds are highly reactive contaminants in a propylene stream. The most common method of removal is by selective hydrogenation which not only xe2x80x9cremovesxe2x80x9d the contaminants but converts them to valuable product propylene. The propylene stream is then fractionated to remove a portion of the stream which contains unconverted MAPD. The stream containing the unconverted MAPD is called xe2x80x9cGreen Oilxe2x80x9d and the fractionation tower used to separate the stream containing the unconverted MAPD from the propylene is called the xe2x80x9cGreen Oil Towerxe2x80x9d.
Prior methods of the selective hydrogenation of MAPD in propylene streams have used both liquid and vapor phase reactors. Generally while conversion is good, in both cases the selectivity drops off rapidly with time. Thus, it would be desirable to improve selectivity in general and keep the selectivity over time. Both the vapor phase and liquid phase system utilize at least two reactors in parallel, with sometimes three, leaving at least one spare for regeneration.
The main drawback of this method is that the selectivity to propylene is not always as desired. The by-product of the hydrogenation, however, is propane which is not considered a contaminant when further processing of the propylene is carried out. However, it does reduce the amount of valuable propylene produced.
The term xe2x80x9creactive distillationxe2x80x9d is used to describe the concurrent reaction and fractionation in a column. For the purposes of the present invention, the term xe2x80x9ccatalytic distillationxe2x80x9d includes reactive distillation and any other process of concurrent reaction and fractional distillation in a column regardless of the designation applied thereto.
A preferred catalytic distillation is one in which a distillation structure also serves as the catalyst for the reaction. The use of a solid particulate catalyst as part of a distillation structure in a combination distillation column reactor for various reactions is described in U.S. Pat. Nos. (etherification) 4,232,177; (hydration) 4,982,022; (dissociation) 4,447,668; (aromatic alkylation) 5,019,669 and (hydrogenation) 5,877,363. Additionally U.S. Pat. Nos. 4,302,356; 4,443,559; 5,431,890 and 5,730,843 disclose catalyst structures which are useful as distillation structures.
Hydrogenation is the reaction of hydrogen with a carbon-carbon multiple bond to xe2x80x9csaturatexe2x80x9d the compound. This reaction has long been known and is usually done at super atmospheric pressures and moderate temperatures using a large excess of hydrogen over a metal catalyst. Among the metals known to catalyze the hydrogenation reaction are platinum, rhenium, cobalt, molybdenum, nickel, tungsten and palladium. Generally, commercial forms of catalyst use supported oxides of these metals. The oxide is reduced to the active form either prior to use with a reducing agent or during use by the hydrogen in the feed. These metals also catalyze other reactions, most notably dehydrogenation at elevated temperatures. Additionally they can promote the reaction of olefinic compounds with themselves or other olefins to produce dimers or oligomers as residence time is increased.
Selective hydrogenation of hydrocarbon compounds has been known for quite some time. Peterson, et al in xe2x80x9cThe Selective Hydrogenation of Pyrolysis Gasolinexe2x80x9d presented to the Petroleum Division of the American Chemical Society in September of 1962, discusses the selective hydrogenation of C4 and higher diolefins. Boitiaux, et al in xe2x80x9cNewest Hydrogenation Catalystxe2x80x9d, Hydrocarbon Processing, March 1985, presents a general, non enabling overview of various uses of hydrogenation catalysts, including selective hydrogenation of a propylene rich stream and other cuts. Conventional liquid phase hydrogenations as presently practiced required high hydrogen partial pressures, usually in excess of 200 psi and more frequently in a range of up to 400 psi or more. In a liquid phase hydrogenation the hydrogen partial pressure is essentially the system pressure.
UK Patent Specification 835,689 discloses a high pressure, concurrent trickle bed hydrogenation of C2 and C3 fractions to remove acetylenes. The selective hydrogenation of MAPD in propylene streams utilizing catalytic distillation alone is disclosed in International Application WO 95/15934.
It is an advantage of the present process that the propadiene and methyl acetylene contained within the hydrocarbon stream contacted with the catalyst are selectively converted to propylene with very little if any formation of oligomers or little if any saturation of the mono-olefins contained in the feed.
The present invention comprises the selective hydrogenation of methyl acetylene and propadiene (MAPD) contained within a propylene rich stream to purify the stream and obtain greater amounts of the propylene. In a class of preferred embodiments the propylene rich stream is fed along with hydrogen first to a standard single pass fixed bed reactor and the effluent then fed to a catalytic distillation column reactor and contacted with hydrogen in a reaction zone containing a hydrogenation catalyst, such as supported palladium oxide catalyst, preferably in the form of a catalytic distillation structure. The hydrogenation catalyst in the single pass fixed bed reactor may the same or different from that in the catalytic distillation column reactor. Hydrogen is provided as necessary to support the reaction and, it is believed, to reduce the oxide and maintain it in the hydride state. The distillation column reactor is operated at a pressure such that the reaction mixture is boiling in the bed of catalyst. If desired, a bottoms stream containing any higher boiling material (the Green Oil) may be withdrawn to effectuate a complete separation.
The single pass fixed bed reactor(s) may be any of those known in the art, as may the hydrogenation catalyst.
The hydrogen rate must be adjusted such that it is sufficient to support the hydrogenation reaction and replace hydrogen lost from the catalyst but kept below that required for hydrogenation of propylene and, in the case of the catalytic distillation reactor, to prevent flooding of the column which is understood to be the xe2x80x9ceffectuating amount of hydrogen xe2x80x9d as that term is used herein. Generally the mole ratio of hydrogen to methyl acetylene and propadiene in the feed to the fixed bed of the present invention will be about 1.05 to 2.5 preferably 1.4 to 2.0.
In some embodiments the process may be described as comprising the steps of:
(a) feeding (1) a first stream comprising propylene, methyl acetylene and propadiene and (2) a second stream containing hydrogen to a single pass fixed bed reactor wherein a portion of the methyl acetylene and propadiene react with the hydrogen to produce propylene;
(b) feeding the effluent from step (a) to a distillation column reactor into a feed zone;
(c) concurrently in said distillation column reactor
(i) contacting unreacted methyl acetylene and propadiene with hydrogen in a distillation reaction zone with a hydrogenation catalyst capable of acting as a distillation structure thereby reacting a further portion of said methyl acetylene and propadiene with said hydrogen to form additional propylene, and
(ii) separating the propylene contained by fractional distillation and
(e) withdrawing the separated propylene along with any propane and lighter compounds, including any unreacted hydrogen, from said distillation column reactor as overheads. Optionally the process may include withdrawing any C4 or higher boiling compounds from said distillation column reactor as bottoms. There is no significant loss of propylene from the hydrogenation.